1. Field of the Invention
This invention relates to an integrated varactor with a piezoelectric device for an acoustic ink printhead. In particular, the varactor is integrated with the printhead by placing it directly on the substrate.
2. Description of Related Art
FIG. 1 shows a conventional acoustic ink jet printhead ejector 100. An ink channel 112 is formed in a channel forming layer 110. A Fresnel lens 108 is formed on the surface of a glass substrate 102 and the channel forming layer 110 is bonded to the substrate 102 such that the Fresnel lens is within the ink channel 112. An opening 122 to the ink channel 112 is formed on a top surface 120 of the channel forming layer 110. During normal operation, ink fills the ink channel 112 to form an ink free-surface 114 at the opening 122. A piezoelectric device 31, positioned on the opposite side of the substrate 102 from the ink channel 112, comprises two electrodes 32 and 104 and a piezoelectric layer 106. When an radio-frequency (RF) signal from an RF source 34 is applied between the electrodes 32 and 104, the piezoelectric device 31 generates acoustic energy in the substrate 102 directed toward the ink channel 112. The Fresnel lens 108 focuses the acoustic energy entering the ink channel 112 from the substrate 102 onto the ink freesurface 114. The ink in the ink channel 112 forms an ink mound 116 in the ink-free surface 114. The ink mound 116 eventually becomes an ink drop 118 moving toward a recording medium.
In conventional acoustic ink jet printheads, an RF switch such as a PIN diode or a varactor controls ink ejection by switching the RF signal on and off. Where a varactor is used as an RF switch, the RF signal powers the varactor and the piezoelectric device 31, which are serially connected. In this circuit, the varactor functions as a capacitor switch for the piezoelectric device. When the varactor capacitance is increased above a threshold by increasing a control signal to the varactor, the piezoelectric device 31 activates, causing an ink drop 118 to be ejected from the ink channel 112.
Conventionally, an acoustic ink jet printhead contains an array of the ejectors 100. Because varactors are not manufactured on the same substrate as the piezoelectric device 31, individual varactors are placed onto the printhead substrate and electrically connected to the printhead by wire bonding. Thus, manufacturing conventional printheads not only incurs undesirable assembly costs, but also prevents manufacturing of high density ejector printheads, since space must be allowed for manually assembling the varactors.
FIG. 2 shows a known method for integrating varactors into the printhead. This acoustic ink jet ejector includes a substrate 102, which may be silicon, having an acoustic lens 208. The acoustic lens 208 focuses the acoustic energy from the substrate 102 onto the ink free-surface 114. The lens 208 performs a similar function as the Fresnel lens 108 of FIG. 1. A piezoelectric device 31 and a varactor 10 are formed on the surface of the substrate 102 opposite the lens 208. The piezoelectric device 31 comprises the first electrode 104 formed on the substrate 102, the piezoelectric layer 106 formed on the first electrode 104 and the second electrode 32 formed on the piezoelectric layer 106. The varactor 10 includes a dielectric layer 210, an amorphous silicon (aSi) layer 212, an interface layer 214 and a third electrode 216.
This integrated acoustic ink jet ejector/varactor operates similarly to the ejector shown in FIG. 1. The piezoelectric device 31 is formed directly on the substrate 102 to ensure the acoustic energy generated by the piezoelectric device 31 easily flows into the substrate 102. The varactor 10 is formed on the piezoelectric device 31 on the side opposite the substrate 102.
Placing the varactor 10 on the piezoelectric device 31 requires first forming the dielectric layer 210 on the electrode 32 and then forming the active varactor layer 212 over the dielectric layer 210. Conventionally, aSi is used as the active layer 212 material because the processing temperature for aSi is more compatible with the temperature range that can be withstood by the piezoelectric layer 106. However, because aSi is very resistive, the operating frequency range of the varactor 10 is limited to below the operating frequency range of acoustic ink jet ejectors 100.